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1. Field of the Invention
The present invention relates generally to measurement while drilling and logging while drilling technologies. More specifically, the invention relates to detecting telemetry from downhole sensors in a drilling operation by analyzing interaction patterns between pressure pulses. Drilling engineers have ordinary skill in this art.
2. Description of the Related Art
Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole commonly is referred to as xe2x80x9clogging.xe2x80x9d Logging has been known in the industry for many years as a technique for providing information regarding the particular earth formation being drilled and can be performed by several methods. In conventional oil well wireline logging, a probe is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole.
Wireline logging is useful in assimilating information relating to formations downhole but it has certain disadvantages. For example, before the wireline logging tool can be run in the wellbore, the drillstring and bottomhole assembly must first be removed, or tripped, from the borehole, resulting in considerable cost and loss of drilling time for the driller (who typically is paying daily fees for the rental of drilling equipment). In addition, because wireline tools are unable to collect data during the actual drilling operation, drillers possibly must make decisions (such as the direction to drill, etc.) without sufficient information, or else incur the cost of tripping the drillstring to run a logging tool to gather more information relating to conditions downhole. In addition, because wireline logging occurs a relatively long period after the wellbore is drilled, the accuracy of the wireline measurement can be questionable. As one skilled in the art will understand, wellbore conditions tend to degrade as drilling fluids invade the formation in the vicinity of the wellbore. Additionally, the borehole shape may begin to degrade, reducing the accuracy of the measurements.
Because of the limitations associated with wireline logging, there recently has been an increasing emphasis on the collection of data during the drilling process itself. By collecting and processing data during the drilling process, without the necessity of tripping the drilling assembly to insert a wireline logging tool, the driller can make accurate modifications or corrections xe2x80x9creal-timexe2x80x9d, as necessary, to optimize drilling performance. For example, the driller may change the weight-on-bit to cause the bottomhole assembly to tend to drill in a particular direction. Moreover, the measurement of formation parameters during drilling, and hopefully before invasion of the formation by the drilling fluid, increases the usefulness of the measured data. Further, making formation and borehole measurements during drilling can save the additional rig time which otherwise would be required to run a wireline logging tool.
Techniques for measuring conditions downhole, and the movement and location of the drilling assembly contemporaneously with the drilling of the well, have come to be known as xe2x80x9cmeasurement-while-drillingxe2x80x9d techniques, or xe2x80x9cMWD.xe2x80x9d Similar techniques, concentrating more on the measurement of formation parameters of the type associated with wireline tools, commonly have been referred to as xe2x80x9clogging while drillingxe2x80x9d techniques, or xe2x80x9cLWD.xe2x80x9d While distinctions between MWD and LWD may exist, the terms MWD and LWD are often used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that the term encompasses both the collection of formation parameters and the collection of information relating to the position of the drilling assembly while the bottomhole assembly is in the well. The measurement of formation properties during drilling of the well by LWD systems improves the timeliness of measurement data and, consequently, increases the efficiency of drilling operations. Typically, LWD measurements are used to provide information regarding the particular formation in which the borehole is traversing.
Referring to FIG. 1, there is illustrated an MWD system. A well bore or borehole 54 contains a drillstring or drill pipe 36 which includes a hollow center region, and defines an annulus 44 (the region between the outside of the drill string and periphery of the borehole). Also shown are stand pipe 34, drill bit 42, and transmitter 40. Stand pipe 34 connects above the earth""s surface (or rig floor) 58 to desurger 26, pressure transducer 60, signal processor 62 (through a transmission line 50), mud pump 24 and drillstring 36. Drill bit 42 attaches to drillstring 36 at the lower end of the drillstring. Transmitter 40, part of a bottomhole assembly (not shown in its entirety), is located near the bottom of the drillstring, proximate to drill bit 42.
Typically, a pit at the surface of the earth (not shown) contains drilling fluid or mud. Mud pump 24 forces the drilling fluid into the drillstring, where it flows in a downstream direction as indicated by arrow T. Eventually, it exits the drillstring via ports in the drill bit 42 and circulates upward via annulus 44. The drilling fluid thereby lubricates the bit and carries formation cuttings to the surface of the earth. The drilling fluid is returned to the pit for recirculation.
Transmitter or pulser 40 generates an information signal representative of measured downhole parameters. This information signal typically is a pressure pulse signal that travels along the mud column at the speed of sound. Pulsers are known and typically transmit at low data transmission rates around 1 bps. Other devices are known which are capable of creating the mud pressure pulses. For instance, a mud siren, which typically creates acoustic waves within the drilling fluid in a frequency range of 12 to 24 Hertz, could be modified to generate the drilling fluid pressure pulses this invention is designed to detect. Pressure transducer 60 receives the mud pressure pulse at an upstream location, such as at the surface of the earth and converts the pressure signals to electronic signals. Transducer 60 outputs the received waveform across communication path 50 to signal processor 62 which operates to process and decode the received signals.
In an ideal system, each and every mud pressure pulse created downhole would propagate upstream and be easily detected by a pressure transducer at the surface of the earth. However, drilling mud pressure fluctuates significantly and contains noise because of several drilling parameters. The primary sources of noise in the pressure signal comprise: (1) the mud pump; (2) torque noise; and (3) bit noise. Bit noise is created by vibration of the drill bit during the drilling operation. As the bit moves and vibrates, bit jets where the drilling fluid exhausts can be partially or momentarily restricted, creating a high frequency noise in the pressure signal. Torque noise is generated downhole by the action of the drill bit sticking in a formation, causing the drillstring to torque up. The subsequent release of the drill bit relieves the torque on the drilling string and generates a low frequency, high amplitude pressure surge. Finally, the mud pumps themselves create cyclic noise as the pistons within the mud pump force the drilling mud into the drillstring.
Most drilling systems contain a dampener or desurger 26. The desurger is fluidly connected to the high pressure drilling mud on a drilling mud side 32. The desurger further has a gas or nitrogen side 28 which is separated from the mud side by diaphragm or separation membrane 30. The purpose of the desurger is to reduce noise levels generated by the mud pump 24. Manufactures of desurgers generally recommend the nitrogen side 28 pressure be filled to be approximately 50 to 75% of the operating pressure of the drilling mud. By expansion and contraction of the separation diaphragm 30, the desurger 26 has a variable volume or capacity which tends to absorb mud pressure increases and lessen mud pressure decreases. Though the desurger may reduce noise levels from the pump, significant noise can still be present which yields a poor signal to noise ratio in the detection of pulses created by the pulser downhole. Further, the pulsation dampener is a primary source of signal distortion of mud pressure pulses since the dampener tends to smooth these pulses in the same manner as it does the pressure surges from the mud pump. When high mud pressure pulse rates are used, a pressure buildup in the pipe occurs and the smoothed pulses become even more difficult to identify. Also, the desurgers signal reflective properties are most prevalent when operating in the recommended nitrogen pressure ranges.
Referring to FIG. 2a, there is depicted an idealized series of mud pressure pulses. For illustrative purposes, the time periods between successive pulses is shortened representing higher data rates. FIG. 2b depicts the effect the desurger might have upon the mud pressure pulses of FIG. 2a showing two related phenomena. First, the desurger tends to smooth the sharp rising and falling edges of the mud pressure pulses, similar to the way in which a capacitor might smooth an electrical signal in an electrical circuit. Second, as the pulse rate increases and the subsequent time between pulses decreases, the dampening effect only allows for a partial settling of the pressure before a subsequent mud pressure pulse arrives and therefore a general increase in pressure occurs with a corresponding decrease in detected pulse amplitude. This combination of factors makes increased data rates hard to achieve because of the signal degradation caused by the dampening effect upon the mud pressure pulses as data rates increase.
In addition to noise not absorbed by the desurger and desurger dampening, related art devices have also had to contend with a problem of reflection of upstream traveling pulses from one, both or a combination of the desurger 26 and mud pump 24. What is needed is a mud pulse detection system which is not significantly affected by the presence of the reflected pulses or the smoothing effect of the desurger.
The present invention features two embodiments for detecting drilling fluid pressure pulses generated by downhole or downstream devices by analyzing interaction between upstream traveling drilling fluid pressure pulses and downstream traveling fluid pressure pulses created by reflection of upstream traveling fluid pressure pulses.
In the preferred embodiment, two transducers are used to detect pressure pulses downstream from reflective elements being one or both of a desurger and/or mud pump. The first transducer is positioned such that a leading portion of the upstream traveling pulse is measured before interference of the upstream traveling pulse with a downstream traveling pulse. This location further provides that a trailing portion of the downstream traveling pulse is measured without interference from a trailing portion of the upstream traveling pulse. A second transducer in this embodiment is located as close as possible to the desurger. Inasmuch as the desurger and the mud pump form a reflective element, the fluid pressure pulse detected at this second location by the second transducer has the characteristic of being the almost instantaneous summation of the upstream traveling pulse and the downstream traveling pulse created by reflection of the upstream traveling pulse because the transducer is located so close to the reflective element. Given these two signals, one or both is time adjusted, and then the second measurement is subtracted from the first measurement to give a difference signal. It is within this difference signal that pressure spikes indicating leading and trailing edges of a fluid pressure pulse of the mud telemetry communication can be detected.
A second method comprises the use of one transducer located at the first transducer location of the preferred embodiment. By bandpass filtering the signal received at the transducer, it is possible to detect the presence and width of a mud pressure pulse sent from downhole by looking for those portions of a signal where: (1) the transducer detects the upstream traveling pulse before interference with the downstream traveling pulse; and (2) the transducer detects the downstream traveling pulse after the upstream traveling pulse has passed the physical location of the first transducer.
Thus, the present invention comprises a combination of features and advantages which enable it to overcome various problems of prior devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings.